1. Field of the Invention
The present invention relates to a receiving system for television signals or more in particular to a receiving system for an image signal band-compressed by multiple sub-Nyquist sampling.
2. Description of the Prior Art
In transmitting a broad-band image signal such as a high-definition television signal, a system has been proposed for reducing the signal band width by sub-Nyquist sampling as disclosed in "HD-TV Broadcasting System Using Single Channel Satellite (MUSE)", Technical Report of The Television Society, TEBS95-2, Published on Mar. 22, 1984. In this system, which is called the multiple sub-Nyquist sampling transmission system of the motion compensation type, the sampling phase is offset between fields, between frames and between lines to complete a cycle of sampling phase by four fields, thereby performing transmission of image signals. The receiving side is provided with a frame memory, and it stores image signals transmitted sequentially over four fields. The image signals are combined to restore a television image. A well-known television system of the 2-to-1 interlaced scanning is based on the above-mentioned inter-frame offset sampling processing and the reproducing processing of the image signals.
The system for transmitting image signals by the inter-field and inter-frame offset sampling often has a complicated decoding circuit for receiving these image signals and reproducing a television image. Further, it is practically difficult to detect moving parts of images completely from among the image signal subjected to band-compression processing. Accordingly, it is effective to transmit an auxiliary signal for simplifying the structure of the decoding circuit, namely, to transmit a control signal for the receiving system together with the image signal. In the above-mentioned system, various control signals including a motion vector signal, a motion detection circuit control signal, etc. are transmitted.
The process of receiving a television signal and reproducing an image by the above-mentioned system basically requires a frame memory to be contained in the receiving system. Then, the interpolation processing of dropouts in the input signal may be performed by using this frame memory. A dropout signal interpolation system is disclosed, for example, in "Home Video Disk for MUSE System", Technical Report of The Television Society, TEBS99-4, Published on Sept. 20, 1984, and it has a configuration such as shown in FIG. 3. In FIG. 3, an image signal of 16.2 MBPS supplied to a signal input terminal 1 is applied to an inter-frame interpolation processing loop including a switch 2 and field memories 3 and 4. A delay time of this loop is set to approximately 562 H at the field memory 3 and to approximately 563 H at the field memory 4, amounting to one frame in total, where H denotes one horizontal scanning period. These memories 3 and 4 are subjected to inter-frame interpolation processing by the switch 2 which performs a selective operation at approximate time intervals of 30 nanoseconds, thereby supplying the next stage with a signal of 32 MBPS formed by time division multiplexing of respective image signals having a time difference of one frame therebetween. The changing-over operation of the switch 2 is controlled by a change-over signal 5 (hereinafter called "a subsample clock signal"), which is supplied through a subsample clock control circuit 6 to effect changing over of a movable arm of the switch 2 so as to perform switching of the output of the switch 2 between the image signal supplied to the fixed terminal a of the switch 2 and the output signal of the inter-frame interpolation processing loop supplied to the fixed terminal b of the switch 2, thereby performing an inter-frame interpolation processing operation. When a dropout occurs in an input signal, on the other hand, predetermined signal dropout information (hereinafter called "a dropout signal") is supplied to the terminal 7, and the control signal from the subsample clock control circuit 6 is supplied to the switch 2 in such a manner as to change over the switch 2 to terminal b. As a result, the field memory 3 is supplied again with a signal which precedes by one frame. In this way, the dropout signal is interpolated. An output produced from the inter-frame interpolation loop is supplied to a spatial interpolation processing circuit 8, a motion detection circuit 9 and a mixing circuit 10. The spatial interpolation processing circuit 8 extracts a signal of an associated field from among signals subjected to inter-frame interpolation processing, and, after subjecting them to predetermined spatial interpolation processing, the spatial interpolation processing circuit 8 supplies a moving image portion thereof to the mixing circuit 10. The motion detection circuit 9 detects the moving portion by using the inter-frame correlation of image signals. The moving signal portion thus detected is expanded along the time axis, that is, in the direction of a field or frame, through a temporal filter 14 which is composed of a maximum (MAX) selection circuit 11, a field memory 12 and a coefficient circuit 13, and then an output signal of the temporal filter 14 is supplied to the mixing circuit 10. The mixing circuit 10 performs mixing control of a signal associated with the static portion of an image supplied through conductors 15 and 16 and a signal representing a moving image portion by using a motion detection signal supplied through a conductor 17, and it supplies a reproduced high-definition television signal to a signal output terminal 18.
The above-mentioned well-known receiving system for a band-compression image signal is provided with a memory for inter-frame interpolation processing, so that it is capable of interpolating a dropout portion of an input image signal with a signal, which precedes by two frames, by changing over the switch 2 to the terminal b by using a dropout signal supplied externally from through a terminal 7. This processing permits complete compensation for a static image. However, a rapidly moving image portion, a varying scene, etc. would be deteriorated in the image quality, if it is interpolated by a signal which precedes by two frames.
Further, in the aforementioned system configuration which lacks a dropout compensation circuit, when a dropout occurs in an image signal, a difference between a signal associated with the dropout portion and an output signal, which precedes the dropout portion by two frames, is detected by the motion detection circuit 9 and time-expanded by the temporal filter 14 at the same time. As a result, an undesirable motion signal remains in the dropout portion of the image signal for some length of time from the time of generation of the image signal dropout. In other words, normal determination of a static image and a moving image is not effected for some length of time at the image signal dropout portion, resulting in a problem that, in such a case as mentioned above, a portion, which should be processed as a static image, is processed as a moving image, thereby deteriorating the image quality conspicuously.